Polyethylene (PE) is synthesized via polymerizing ethylene (CH2═CH2) monomers. Because PE is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the synthesis methods, PE can be generally classified into several types such as LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
It is known that the polymerization of olefins, e.g. ethylene, involves the polymerization of olefin monomer with the aid of an organometallic catalyst of Ziegler-Natta and a co-catalyst. Catalyst systems for polymerization and co-polymerization of olefins known as Ziegler-Natta systems consist on the one hand, as catalyst, of compounds of transition metals belonging to Groups IV to VII of the periodic table of elements, and on the other hand, as co-catalysts, of organometallic compounds of metals of Groups I to III of this Table. The catalysts most frequently used are the halogenated derivatives of titanium and vanadium, preferably associated with compounds of magnesium. Moreover, the co-catalysts most frequently used are organoaluminium or organozinc compounds. When the catalyst is highly active, especially when it is employed in the presence of a large quantity of co-catalyst a formation of polymer agglomerates, which may be considerable, can be observed. In a typical Ziegler-Natta catalysis system the monomer, e.g. ethylene or propylene, is bubbled into the suspended catalyst and the ethylene or propylene rapidly polymerizes to a high molecular weight linear polyethylene or polypropylene. A characteristic of all Ziegler-Natta catalysts is that they all yield straight chain polymers.
The use of Ziegler-Natta-catalysts in a polymerization method has been improved over a number of generations since the initial work by Ziegler and Natta in the 1950s. Seeking to increase both the activity and the stereoselectivity has been the driving force for the continuous development of the catalyst system. In addition to the support material, this comprises as actual catalyst a transition metal compound, e.g. a titanium compound, which is activated only by addition of an aluminium-containing co-catalyst.
In employing Ziegler-Natta catalysts, it has been customary to inject the catalyst as a slurry in a diluent into a reaction zone of the reactor and to introduce also the olefins being polymerised. Several methods for supplying catalyst to a polymerisation reactor have been described in the prior art.
U.S. Pat. No. 3,846,394 describes a process for the introduction of Ziegler-Natta catalyst slurry in a reactor. The process comprises the preparation of Ziegler-Natta catalyst slurry, the transfer of the slurry via a feed conduit from a storage zone to a metering zone, and the introduction of the slurry into a reactor. In order to avoid the back flow of monomer and other contents of the reactor into the Ziegler-Natta catalyst conduits the process provides the catalyst feed conduit to be flushed with a diluent inert to the Ziegler-Natta catalyst, said diluent being introduced into said conduit downstream of the metering zone.
WO 2004/026455 discloses a catalyst slurry feeding assembly for a polymerization reactor. Catalyst slurry is prepared in a mixing tank and subsequently fed to one or more storage tanks. The storage tanks include agitators so that the catalyst slurry is maintained at an essentially homogeneous solids-to-liquid ratio. From the storage tanks, the catalyst slurry is pumped to the polymerization reactor along a fluid passage having a flow meter. The flow of the catalyst slurry can be continuous and/or adjusted based on a measured parameter. A major drawback of the presented system is that it does not involve the introduction of catalyst to a polymerization reactor which has been pre-contacted with a suitable co-catalyst.
It is well known that the polymerisation reaction is quite sensitive to the quantity of catalyst utilized, and it is also known that the amount of catalyst added to the reactor is based on the flow rate of the catalyst to the reactor. However, one of the major problems in the injection of Ziegler-Natta catalyst slurry in a diluent to a reactor in prior art methods is that it is difficult to control the amount of Ziegler-Natta catalyst injected. Also, the catalyst tends to clog catalyst injection means such as pumps and the like and lines carrying the slurry.
For instance, U.S. Pat. No. 3,726,845 describes the supply and control of the amount of catalyst and the maintenance of the catalyst line and pump free by alternately feeding catalyst slurry and diluent to the reaction zone, allowing careful control of the amount of catalyst and control of the cleanliness of equipment such as lines and pumps and freedom from clogging.
GB 838,395 relates to a process and apparatus for producing a slurry of a solid catalyst in hydrocarbon diluent for use in a chemical reaction. The process comprises preparing concentrated catalyst slurry in a hydrocarbon diluent and admixing said concentrated slurry with additional diluent and introducing said admixture to a reaction zone. According to the process, the specific inductive capacity of the slurry is continuously determined prior to the introduction of same to said reaction zone, the inductive capacity of the slurry being dependent upon the concentration of catalyst in the slurry. The process further comprises regulating the ratio of concentrated slurry to added diluent responsive to variations of said specific inductive capacity from a predetermined value so as to maintain a slurry of substantially constant dielectric value.
Moreover, another problem relates to catalyst supply is that it has been difficult to control Ziegler-Natta catalyst flow rate in an adequate way. Ziegler-Natta catalyst flow rate is generally fixed for a certain operation and catalyst feeding systems do not account for variations in the feed flow rate.
Certain systems have been disclosed wherein the catalyst flow rate can be controlled and adapted. For instance, U.S. Pat. No. 5,098,667 discloses a system wherein a continuous flow of particulate solids is supplied to a polymerization reactor by continuously pumping a dilute catalyst slurry to the reactor. The flow rate of the dilute slurry is continuously manipulated so as to maintain a desired flow rate of solid particles contained in the dilute slurry to the reactor. In another example, U.S. Pat. No. 4,619,901 discloses a system wherein unreacted monomer concentration is maintained in the reaction effluent removed from a polymerization reactor by manipulating the catalyst feed rate to the polymerization reactor. A drawback of these above-mentioned systems is that these systems are rather complicated and rely on an adequate measurement of catalyst slurry characteristics and polymerization conditions.
Another problem relating to the field of catalyst supply to a reactor consists of supplying a co-catalyst during a polymerisation reaction. It is known that the activity of certain Ziegler catalyst systems can be improved by increasing the quantity of organometallic compound used as the co-catalyst. In this case, it is generally necessary to employ in the polymerization medium relatively large quantities of organometallic compounds as co-catalysts. However, this provides disadvantages including safety problems, related to the fact that these organometallic compounds spontaneously ignite on contact with air.
A number of techniques for the introduction of the co-catalyst has already been proposed. One technique consists for example of introducing the co-catalyst directly into the polymerization reactor. However, such methods do not allow bringing co-catalyst into contact with the Ziegler-Natta catalyst before entering the reactor, although such pre-contact is particularly desirable in order to provide effective Ziegler-Natta catalyst-co-catalyst mixtures.
Another technique consists of contacting the catalyst and co-catalyst before their introduction into the polymerization medium. For instance, U.S. Pat. No. 3,726,845 discloses a system and method wherein a Ziegler-Natta catalyst is brought into contact with an aluminum alkyl co-catalyst prior to be delivered upon mixture with a diluent to a polymerization reactor. In this latter case, however, it is difficult to control the pre-contact time of the catalyst with the co-catalyst. Moreover, insufficient pre-contact of a catalyst with its co-catalyst may induce the formation of undesired waxes.
It is therefore a general object of this invention to provide an improved method for optimising catalyst introduction in a polymerisation reactor. It is an object of the present invention to optimise the supply of a Ziegler-Natta catalyst to a polymerisation reactor wherein polyethylene is prepared. More in particular, the present invention also aims to provide a method enabling to effectively control the flow rate of a catalyst, and in particular a Ziegler-Natta catalyst, to a polymerisation reactor wherein polyethylene is prepared.
It is another object the present invention to provide a method for supplying catalyst, and in particular a Ziegler-Natta catalyst, in pre-contact with a co-catalyst, to a polymerisation reactor, wherein polyethylene is prepared. The invention also aims to provide a method for preparing and supplying an optimised Ziegler-Natta catalyst-co-catalyst mixture to a polymerisation reactor.
Furthermore, the present invention alms to provide a device for preparing catalyst slurry, in particular a Ziegler-Natta catalyst and for supplying said catalyst slurry to a polymerisation reactor in a controlled and efficient way. Moreover, another aim of the invention is to provide a device for preparing a suitable catalyst-co-catalyst mixture and for supplying said mixture to a polymerisation reactor in a controlled and efficient way.